OCE 1001 Lecture: Ocean Circulation

welcome back to an introduction to oceanography from the cengage learning textbook essentials of oceanography written by Tom garrison and Robert Ellis this is the eighth edition and now we're taking a look at ocean circulations in the last lecture we looked at atmospheric circulation what makes the wind blow and what impact does the blowing wind have on the ocean now we're gonna take a closer look at the ocean and how it moves not just those surface currents that we may be familiar with but what's happening deep down in the ocean we'll start out with our main concepts alright our first concept ocean circulation is driven by wind and the differences in water density second we'll look at the fact that surface currents are when driven movements of waters but also driven by density differences between temperature and salinity we'll talk about what a Geyer is and how the Coriolis effect creates those gyres large surface currents that move in circular patterns on the ocean surface we're going to get into the El Nino and the Southern Oscillation and why they are exceptions to the normal wind and current flow we'll talk about water masses that form at the ocean surface and how they retain their distinct properties as they sink and sort into identifiable layers so first and foremost ocean currents are driven by winds the westerlies and the trade winds are two of the winds that drive the ocean's surface currents and you can see the westerly is going to blow from west to east at the surface and the trade winds from east to west and what that allows to happen is that you start to get a clockwise flow in the Northern Hemisphere and then a counterclockwise flow in the southern hemisphere what that ends up actually looking like are these large what are known as gyres or at which are actually going to be the the currents of the ocean turning in a clockwise flow in the Northern Hemisphere and then a counterclockwise flow I down here in the southern hemisphere and each of the the main ocean basins has one of these large cars one of these circular flows so about ten percent of the water in the world ocean is involved in surface currents only about ten percent water flowing horizontally in the uppermost 400 meters of the ocean driven mainly by wind friction so winds driven by uneven solar heating again remember at the equator we heat up at the poles we cool off so air rises at the equator moves north and sinks at the poles and moves back South so winds are driven by uneven solar heating of the Earth's surface they spended the Coriolis effect and drive the movement of the ocean surface currents the prime movers are the powerful westerlies and the persistent easterly's those northeast and southeast trade winds surface currents travel at a speed no greater than about 3% of the wind speed so if you have winds moving a nutter miles per hour of the ocean the surface currents only move there at about 3 miles per hours so the wind really has to be lots and lots of wind to create these surface currents lots of persistent wind so what are the effects of those ocean currents they transfer heat from the tropics to the polar regions they influence weather and climate and they also distribute nutrients and scatter organisms around the globe so a combination of four forces the sun's heat surface winds Coriolis and gravity circulates the ocean surfaces clockwise and the Northern Hemisphere and counterclockwise in the southern hemisphere forming the gyres that we just talked about so in this illustration well for those are the two guards that I showed you in the last slide in this illustration the North Atlantic gyre is a series of four interconnecting currents with different flow characteristics and temperatures you have the North equatorial current you have the Gulf Stream the North Atlantic Current and the canary current and these currents are driven by the Northeast trade winds and the westerlies and the result in this one large Dyer in the North Atlantic so in the northern hemisphere the topmost layer of the ocean will flow at about 45 degrees to the right of the wind direction so it's the same in the southern hemisphere but it's to the left of the wind direction so let's just talk about the northern hemisphere where Coriolis will deflect the ocean current about 45 degrees to the right of the way direction surface water blown by the winds at Point a will then veer to the right of the initial path and continue eastward and then water at point B veers to the right here's B and continues westward and again creating that ocean Geyer that we see in the major ocean basins in this veering of 45 degrees is known as Ekman currents or the Ekman model or the Ekman spiral depending on how you you want to define it so again the wind of the surface is blowing in this direction and because of Coriolis the surface water blows at about 45 degrees to the right of it but each additional layer deeper and deeper in the water and the water also blows to the right or moves to the right of the layer above it so as you go deeper and deeper those ocean currents continue to move to the right of the initial flow until you get to the very very bottom layer which almost is moving in the opposite direction so a Bottura body of water can be thought of as a set of layers the top layer is driven forward by the wind and each layer below is moved by friction each succeeding layer then moves with a slower speed and an angle to the layer immediately above it to the right in the northern hemisphere left in the Southern Miss here until water motion becomes almost negligible through the direction although the direction of movement varies with each layer in the stack the theoretical net flow of the water in the northern hemisphere is about ninety degrees to the right of the prevailing wind now the previous slide we talked about 45 degrees that is going to be for the surface water when you consider all the water movement in the entire deep layer in which the the movement is affected the net effect is about a 90 degrees so at the surface it's about a 45 degree flow but the net effect is about a 90 degree flow to the actual wind motion and again this is known as the Ekman spiral the effective Ekman spiraling and the Coriolis effect caused the water within the Gayo to move essentially in a circular pattern this is the Northeast trade wind this is the water at point B the direction of is about 45 degrees and then it continues to be deflected to the right the water continues clockwise it does not continue clockwise into a spiral but instead generally moves in that circular motion around the basin the movement of water away from point B is influenced by the rightward tendency of the Coriolis effect and the gravity-powered movement of water down the pressure gradient that gravity-powered movement of water down the pressure gradient occurs because this Ekman spiraling causes water to pile up in the center of the guy'll Guyer and so water wants to float down that pile downhill off that pile there is a hill of water and in each ocean basin there's a hill of water in the North Atlantic formed by the Ekman transport the surface of the North Atlantic is raised through wind motion and Ekman transport to form hello Hill so water turns clockwise in inward from the dome then descends under the dome depressing the thermocline so you can see it depresses down that thermocline water movement from B turns westward and flows along the side of the hill the westward moving water is balanced between the Coriolis effect which would turn the water to the right and the flow down the pressure gradient which would turn it which would wanted to cause it to go to the south which is driven by gravity thus the water and a Geyer kind of moves around the edge of the ocean basin so it's combination of the equina qin current turning it to the right combined with this downhill movement turning it south that just causes it to generally and gradually move in a clockwise flow around the ocean basin so another way to look at this surface current flowing around the periphery of the ocean basin again the water at point B is being blown from the north and east by the trade winds that's want to deflect it to the right by the Coriolis effect continue deflection by the Coriolis effect would theoretically then cause that water to spiral in if there were no other impacts on the water but the other impact of the water is the fact that that inward turning that right turning causes the water in the middle of the ocean to pile up it's a small but it's there nonetheless and there is a gradient or a pressure gradient or forced down that hill so that is this arrow from north to south the arrow from south to north is the Coriolis effect and the net effect that the water is to be deflected to the west and then basically around that ocean basin now because the Coriolis effect is stronger to the north than it is to the south that hill is also offset to the west and this is what that looks like in terms of the hill being offset to the west without the Coriolis being stronger to the north than it is to the south you'd have this nice round circle around the ocean basin but because Coriolis increases from the equator to the North Pole the Coriolis effect is going to be stronger in the North Atlantic because it's stronger in the North Atlantic the water is deflected in a greater force to the right in the North Atlantic causing this quicker turn so you're north that want Atlantic flow is a much broader flow it is much wider across the eastern side of the Atlantic Basin whereas here there is less of a turn to the north and it in the in the water and gets sort of piled up along the edge of the basin the effect of this broader flow around the east side and a narrow flow or on the west side the water flowing in a broader channel essentially will move more slowly the water flowing in a narrower channel will move more quickly and this is known as Western intensification without the Coriolis effect the guys will look like this with the Coriolis effect and the fact that the Coriolis effect is strongest in the North Atlantic it makes a stronger turn to the right and it turns more quickly than at the equatorial Atlantic so what ends up happening is you get this Western intensification an equal amount of water flows all the way around the gyres so the water has to move more quickly on the west side in this narrower channel then in the broader channel on the east side and that is known as westward intensification and again here's another look across section of the of the flow of the North Atlantic known as the geostrophic flow which is the model of flow the Gulf Stream is a western boundary current so the Gulf Stream is on the west side of our mouth on the west side of the basin it is a western boundary currents on the west side of the basin it's narrow and it's deep and it carries warm water very rapidly to the north the canary current is on the east side of the basin and the east side of the slope it is a broader wider slope so it's much shallower it's an eastern boundary current a shallower current a wider current that carries cold water at a much more leisurely pace to the south although this gradually sloping hill is only about six six and a half feet high it really wouldn't be apparent to anyone traveling from coast to coast it is large enough to help steer the gyres in each Basin in this case the North Atlantic gyre so what does this look like the surface currents around the ocean basin two things are going to happen due to Western intensification you're going to get warm water flowing up the west side of a basin and cool water flowing down the east side of the basin in the northern hemisphere that's one thing that's going to happen that really three things the second thing is going to be the western boundary currents going to be narrow and stronger the eastern boundary currents going to be weaker and that Hill we talked about in the center of the Atlantic Ocean is offset to the west the actual hill is going to be offset to the west if we look back here the hill is offset away closer to the western boundary current we see that here as well with the center of the G strop it killed being offset to the west so the average height of the surface of the North Atlantic is shown in color in this image from the Topaz beside and satellite red indicates the highest surface green and blue the lowest so you can see that this hill is offset to the west with the western boundary current the Western intensification and really this hill is only about two meters or about six six and a half feet above the surface so you really wouldn't notice it if you're traveling across the Atlantic Ocean but it's enough of a hill to cause that guy to go very much around the Atlantic Ocean in a circle all right there are six great gyres there is the North Atlantic eye which we've talked a lot about the South Atlantic gyre specific South Pacific and then the Indian Ocean gyre and you can see how the west side of the northern hemisphere carries warm water north because we're looking at here are our sea surface temperatures here's the North Atlantic the canary current bringing cool water south in the Pacific Ocean you've got the Kuroshio Current carrying warm water north and then the California Current carrying cold water south here you have in the southern hemisphere it's the opposite where warm water it's traveling toward the poles here in cool water it's traveling toward the equator here same thing in the South Atlantic warm waters moving south cool waters moving north and then in the Indian Ocean you have that warm water moving south and cool water moving to the north so this is an illustration of sea surface temperatures not the heights like the previous slide but temperatures showing the general direction and pattern of the surface current flow the purple color around Antarctica and west of Greenland indicates water below zero Celsius the freezing point of fresh water note that the distortion of the temperature patterns that we would expect from the effects of solar heating alone that has just warm of the Equator that warm water is twisted northward of the Northern Hemisphere and counterclockwise and in southward in the southern hemisphere alright so this is what those six great currents look like and of course the arrows indicate the temperature of the water at the Kuroshio carrying warm water north the California Current carrying so cool water south Gulf Stream warm water north and the North Atlantic taking that water all the way over to Western Europe and this is the reason why Western Europe it may be chilly up there at times but it's not nearly as cold as the land masses at the same latitude over North America because it is that warm ocean current carrying warm ocean water and air to that region again the Bengal current in South Africa is getting cold water to the equator the Brazil current is carrying warm water South the East Australian current is carrying warm water south the Peruvian current stream cold water north there's the Kuroshio of the California as we we said and then there's more last major current in the ocean and that's the entire circumpolar current also known as a West drift it travels westward around the entire globe there's no land master deflected so it just travels around the globe so gyres our imbalance between the pressure gradient that's the pressure gradient going down the hill and the Coriolis effect the pressure gradient once secured the water down the hill the Coriolis effect wants to turn to the right in the northern hemisphere the left in the southern hemisphere and the effect is the guy on of the sixth great ocean currents fiber gyres and this chart shows the names in a usual direction of those major ocean currents and you can see the powerful western boundary currents flow on the west boundaries of the ocean basin and the eastern boundary currents in the east sides of ocean basins and the Gulf Stream is the largest and most powerful current in the world and large summer's peril for western boundary current as well alright so western boundary currents these are the narrow deep and fast currents found in the western borders of ocean basins with well-defined boundaries that are sometimes create actual eddies western boundary currents show move that is warm water poleward if the Gulf Stream the Japan Crowes your current and the Brazilian Current and we also have eastern boundary currents these currents are cold shallow and broad and their boundaries are not very well defined they do not create Eddie's and eastern boundary currents move cold water toward the equator like the canary the Bangala and the California Current tribe transverse currents that is move east to west and west to east and they link the eastern and western boundary currents so to complete that guy you have to have something that links the eastern boundary in the western boundary currents and those are the strands transverse currents so the equatorial currents and the Antarctic circumpolar current are the trends the transverse currents and then you have counter currents and under currents and counter currents will flow opposite to the main current and under current will flow beneath the main flow and they may be flowing with the flow or counter to the main flow and here we have the equatorial a current which is a it's going to be a counter-current that's one example of that all right we mentioned when we were talking about western boundary currents that sometimes they create Eddie's and this is what those Eddie's look like where this western boundary current in the North Atlantic is the strong Gulf Stream and there's the west side of the current and the east side of the current both are very very well defined and as that water moves north the cold water that's moving back down to the south sometimes gets entrained into the western boundary current and it creates these eddies where warm water will spin off and cold water will get pulled in and so you start out with the typical current the cold current moving south or warm current moving north and then some of that cold water gets entrained and so they get a little pool of warm water that gets displaced and that cold water gets pulled into the actual current and sometimes it then gets ejected out of the eastern boundary of the current so the western boundary of the Gulf Stream is usually distinct and marked by abrupt changes in water temperature speed and direction meanders are these Eddie's will form as the Gulf Stream leaves the u.s. coast at about Cape Hatteras right about here the meander can pinch off and eventually become isolated cells of warm water between the Gulf Stream and the coast likewise cold cells can pinch off and become entrained in the Gulf Stream itself the general surface circulation of the North Atlantic so the numbers indicate the flow rates and these are this very odd Swedish word s ve Rd are ups the sphere drops so one sphere drop equals 1 million cubic meters of water per second and you can see that the Gulf Stream is 26 to 55 million cubic meters of water per second so the sphere droop again that's that that word s ve RDR ups moves a tremendous amount of water in the Gulf Stream and you can see this lesser amount of waters in different currents the unit is used to express the volume and transport in ocean currents and it's named in honor of this is Harold who was one of the century's pioneering oceanographers all right the main effect of ocean currents is to transfer heat from the tropical regions to polar regions which generally and greatly affects climate at the mid latitudes the influence on weather and climate can be seen because San Diego has those cool foggy summers Mark Twain once said the coldest summer that should say the coldest winter he ever spent was one summer in San Francisco and of course Washington DC has hot humid summers because the Gulf Stream that moves just by it and so the to influence is the two main impacts of ocean currents are to transfer heat from the tropical to the polar regions and also to distribute nutrients and scatter organisms around the ocean so we talked about San Francisco being called the big guy ER in the North Atlantic moves that cold Northern Pacific waters south in the California Current that eastern boundary current keeping California nice and cold where the Atlantic Gaya the North Atlantic gyre brings warm moist water north with the Gulf Stream keeping the eastern seaboard very warm so these are generally the summer air circulation patterns on the east and west coast United States and warm ocean currents are shown in red the coldest and ocean currents are shown in blue and airs chill as it approaches the west coast and warm as it approaches the East Coast when can also cause vertical movements of ocean water so we talked an awful lot now in the last few minutes over how when induces horizontal movements or these gyres of the ocean currents but when can also induce vertical circulations the vertical movement induced by wind driven horizontal movement of water so as the water moves horizontally across the ocean because of the wind it can create either upwelling or downwelling and upwelling is the upward motion of water it brings cold nutrient-rich water toward the surface and downwelling is the downward motion of water which supplies the deeper ocean with dissolved gases and nutrients so this is what equatorial upwelling might look like the South Equatorial current especially in the Pacific straddles the geographic equator water north of the equator veers the right northward and water south of the equator veers to the left southward that means the surface water diverges so if you look right at that zero degree latitude right in here to the north it moves to the north to the right the South it moves to the south of the left so the water here is diverging what are the mass here is moving away so it has to be replaced and it's replaced by this upwelling this upwelling surface water therefore diverges causing upwelling most of the up weld water comes from the area above the equatorial undercurrent had a depth of 100 meters or so so it brings this water upwards so that's one example of how horizontal movement of water at the surface caused by wind creates a vertical movement of water at the surface and this is another way that wind can induce upwelling we'll talk about the the California Current and win from the north and along that California Current blows north to south it blows north to south well Ekman transport turns it to the right and as it turns it to the right it pulls all this water here offshore and because it's moving all this water offshore that water has to be replaced so it's replaced from deep below creating upwelling so that's coastal upwelling in the northern hemisphere coastal upwelling can be caused by winds from the north blowing along the west coast of the continent water has moved offshore by Ekman transport and is replaced by cold the nutrient Laden water in this diagram the temperature of the ocean surface is shown is in degrees Celsius and so what's very important about this is off the coast of California that Ekman transported that or that offshore movement of water brings very cold nutrient-rich water up along the coast of California and that's why California is well known for its own not only its kelp beds but it's its small baitfish its sardines and it's smelt and its grunion in different types of fish because they come to the sir because all that nutrient-rich water is there and of course that feeds larger and larger fish all right you can also get downwelling I can also occur so if the wind is blowing south along the northern hemisphere west coast for a long period of time so a south wind you're going to get the water moving northward and therefore turning to the right and that's going to pull water off the Atlantic or onshore when it hits the coast it goes down so areas of downwelling that's gonna be warmer water will be low in nutrients and relatively low in nutrient and biological activity at all but it can supply the ocean with dissolved gases meaning there's a lot of ocean atmosphere interaction where oxygen carbon dioxide nitrogen will dissolve into the surface of the ocean and if this south wind creates a right turn because of Ekman kernan or Eggman transport it pulls water onshore and forces water down so that warmer water at the surface gets forced down taking dissolved gases below so again a south wind creating a South current Ekman transport forcing water toward the coast and then downwelling occur so when's creating horizontal movement of water creating vertical movement of water all right so we're talked a little bit about what's known as El Nino and of course there is also La Nina but I'll tell you right off the bat La Nina is kind of a young some of the media created really what we have is El Nino and then non El Nino years and the El Nino is tied to something called the Southern Oscillation so sometimes El Nino is known as ENSO which is El Nino en Esso Southern Oscillation so what we're looking at is a a non El Nino year so this is this on the right hand side is the normal situation across the Pacific Ocean so what you're looking at is the sur Pacific equatorial regions and in the Pacific equatorial regions because we have a northeast trade wind and a southeast trade win that equatorial transverse current moves from east to west it moves from east to west and so what that causes is air to rise off the surface in the West because you get nice warm water over here and sink toward the surface back in the East because you can have colder water here remember cold water is brought in from the north from the south that is with the Peru current California Current brings cold water in from the north here cold water the air sinks over that cold water warm water here it rises over that warm water and so the typical non El Nino here has a lot of warm water piled up on the west side of the Pacific Ocean and cloudy wet weather where we have dry weather on the east side in a non in an El Nino year that is the southern oscillation occurs meaning the pressure patterns this low here in this high here they flip and you you get a little bit of a difference with the low forming sort of in the middle of the Pacific Ocean so let's really through this in an El Nino year when the Southern Oscillation develops the trade winds diminish and then reverse so what happens is the trade winds that are blowing in here they we can reverse and that allows all this warm water instead of flowing to the west all that warm water flows back to the east and where all the warm water in a not only new year is piled up on the west side of the Pacific with cold water over here in an El Nino year because the trade winds flip or reverse that's the oscillation the winds blow back down that gradient and you get much much warmer water over the central equatorial Pacific Ocean in an El Nino year when the Southern Oscillation develops the trade winds diminish and then reverse leading to an eastward movement of warm water along the equator that is the eastward movement right there the warm water along the equator the surface waters of the central and eastern Pacific differ or become warmer and storms over land may actually increase in a non El Nino year normally the air and surface water flow westward and an out blowing of cold water occurs along the west coast of Central and South America so this is the El Nino year with this one water flowing back toward the central and eastern Pacific and an L knee in a non El Nino year where all that warm water stays back here on the west side of the Pacific Basin this is what the kind of the periodic or how often those events happen this goes back to about 1950 and we see the the different El Nino years so 1987 we get a big El Nino year and and in 1998 we had a big El Nino year and you see that these El Nino periods may last for three or four to seven years and then the non El Nino so the equatorial Pacific goes from being warmer than normal to colder than normal and warmer than old and colder than normal here again 1998 much warmer than normal event in through 2000 colder than normal and then we had a period of warmer than colder and so that is sort of the the period of El Nino and non El Nino years the impact of an El Nino season is the warming of the equatorial waters of the Pacific Ocean so the region actually south of Hawaii and that warming that equatorial water creates a much stronger persistent jet stream moving across this part of the Pacific Ocean and with the low pressure that sets up south of Alaska it just produces a very very wet and stormy pattern that moves across essentially the southern tier of the United States this strong wind also moves all the way across the Gulf of Mexico and into the Atlantic Ocean and during an El Nino year this strong subtropical jet actually helps to reduce the number of hurricanes that form or if hurricanes do form it tends to deflect them out into the Atlantic Ocean quickly that is in the Atlantic Basin all right when we're talking about non El Nino the years that La Nina mention but non El Nino years your more typical pattern is a large blocking high pressure area south of Alaska that causes the the main storm track to be deflected way to the north that keeps it much dry across the southern states and even warmer for us across the southeastern states and this jet stream is much more variable so in often times it comes in way to the north and so that subtropical jet aid does not bring strong showers and storms into southern tier but it also doesn't deflect or thwart the development of tropical systems in the Atlantic Ocean so La Nina are non El Nino years we typically have a more active Atlantic hurricane season all right so we talked a little bit about the vertical motion of water in the Atlantic due to wind we talked a lot about the horizontal motion of water due to wind the gyres and the ocean currents let's talk about how density differences cause water to move and those density differences create what's called the thermohaline circulation so thermo being temperature hay line being solved and what it means is the difference in temperature of the ocean water or the difference in salinity of the ocean water causes different density characteristics of layers of water which causes the ocean become density stratified meaning the densest water will be at the bottom and the least dense water will be the top so because of these density differences there are five basic water masses you have your surface water central intermediate deep water and then your bottom water and of course the densest is going to be the bottom water and the least dense is going to be the surface water the coldest and salty is the bottom water and the clearest and least salty and the warmest the surface water so deep water forms and then down wells and polar regions meaning when water gets up to the polar areas it gets very very cold and begins to turn to ice well when ice forms only the pure water transition has a phase change from liquid to solid so of all the water in the polar regions that may have salts in it as that water turns to ice the salt is left behind which means the ice forms leaving behind very cold very salty water that tends to download or sink so deep water forms and down wells in the polar regions it becomes Antarctic bottom wire water or North Atlantic deep water and this formation and movement of deep water definitely affects the climate all right so this global circulation creates a global heat connection so let's let's go through this this is what's known as the great global conveyor belt essentially the global pattern of deep circulation resembles a vast conveyor belt that conveyor belt carries surface water to the deep depths and it does so in the polar regions and then back up to the surface again in the equatorial regions so beginning with the formation of that North Atlantic deep water so that's that water in the North Atlantic that freezes becomes very salty what's left over becomes very salty cold and dense and sinks forms that North Atlantic deep water and that's going to happen again up in the North Atlantic so that's the water that comes up it freezes and it sinks for creating this blue arrow here at the North Atlantic deep water which then flows south under the force of gravity water north of Iceland that's what we're talking about this water mass then flows south along the Atlantic and then flows over and mixes with the deep water formed near Antarctica so it flows south and mixes with this deep water near Antarctica the combined mass that's the to the North Atlantic deep water and the Antarctic deep water bottom water the combined mass then circumnavigates goes all the way around Antarctica and then begins to move north into the Indian Ocean and also in the Pacific basins water gradually warms here and mixes upward to be returned to the North Atlantic by surface circulation so it mixes upward in the Atlantic flows are on that guy gets caught up in the southern guy maybe flows around South America into the South Atlantic and eventually up into the North Atlantic so you start out very cold you drop at the bottom you mix with the Antarctic water you move up in the Indian Ocean you move over into the Pacific Ocean you rise back to the surface as you warm that eventually you connect back up into the North Atlantic where the whole process starts over again and the whole very slow-moving system very important for moving Heat north and cold back to the south it's called the great ocean conveyor and it's one of the most powerful forces on the planet it's made up of warm currents which travel at the surface whilst at the bottom of the ocean are much colder currents the conveyor transports oxygen nutrients and warmth around the world [Music] it's vital to the health of all life on Earth [Music] yet the conveyor wouldn't keep flowing if it wasn't for what happens in the cold northern oceans [Music] this is where the surface water sinks to join the deep water currents [Music] the sinking happens because the waters very cold this makes it dense and heavy so it plunges to the bottom of the ocean and it's the sinking water which keeps the entire conveyor moving as it travels south it hugs the ocean floor until it warms at the equator where it eventually rises to complete the circuit without cold water sinking at the polls the ocean conveyor would collapse the sea would no longer be supplied with oxygen and nutrients it would become stagnant and lifeless it takes roughly a thousand years for water to circulate all the way around the conveyor and this global network of currents controls a well-being of the entire planet without it a world simply wouldn't function so a couple different illustrations of how the water travels not only across the surface but also over the seabed at the left the water layers and deep circulation of the Atlantic Ocean here so we see where water sinks in the northern parts of the Atlantic Ocean of near Greenland and Iceland has a salty water turns to ice leaving behind very salty very cold water which is more dense and so it sinks it sinks down at the bottom of the Atlantic Ocean and then flows south until it gets down to about Antarctica where the same processes cause water to sink in Antarctica the water turns isolating behind cold salty water it sinks those two masses mix the Antarctic bottom water in the North Atlantic deep water they mix and then they travel either over into the Indian Ocean or the Pacific Ocean and then some upwelling and warming brings them back up to the surface creating the entire circulation and so to the right you have a model of the thermo thermocline circulation caused by heating in the low latitudes so here you heat the water up becomes less dense and then chilling in the higher altitudes that causes the water to sink and again so these differences in both so entity and in temperature create density differences that cause this water to move around the globe the global conveyor belt and here's a very simplified view of the thermo aline both heat and salt or heat and salinity temperature and salinity circulation in the Atlantic surface water becomes dense and sinks at the north and south polar regions being denser Antarctic bottom water slips beneath north Atlantic deep water and the two kind of mixes to some degree there the water then gradually Rises across a very large area in the tropical temperate zones then flows poorer to repeat the cycle as it talked about the text fresh water arriving in the North Atlantic from rapidly milk and polar ice could slow the formation of the North Atlantic deep water with big implications in the climate here so what that's telling us is that if the climate warms to a degree that the polar regions see a great deal of ice melt then all that fresh water that would come from the ice melt would then cause a North Atlantic deep water to to form more slowly because there wouldn't be the density differences and that could cause some big impacts in terms of chilling the climates of Europe so that's one of those interesting feedback mechanisms where the warming of the globe may cause the melting of polar ice but the melting of polar ice actually slowing down the global conveyor belt and therefore cooling Western Europe in Chapter eight we've learned that ocean water circulates in currents and that surface currents affect only the uppermost 10% of the world's ocean the movement of surface currents is powered ultimately by the warmth of the Sun but it's the warmth of the Sun and the differential heating of the Earth's surface that causes the winds and the winds are what effectively caused the ocean currents to begin moving water and surface currents tends to flow horizon-- label we also learned that horizontal movement of water along the coastline or near the equator can actually cause a vertical response in water so horizontal movement of water because of winds can create vertical movements of water surface currents also transfer heat from the tropics to the polar regions and they influence weather and climate and again one of the primary ways we see that is with the Gulf Stream transitioning very warm water into the North Atlantic that keeps Western Europe much warmer than it might be at that latitude the ocean currents also distribute whether they're horizontal or vertical distribute nutrients and scatter organisms and they have also as we and chapters 1 & 2 contributed to the spread of humanity to remote islands and of course important factors in maritime commerce so the other 90% of the ocean water that lies below the surface zone is driven by the force of gravity and density differences in water layers as dense water sinks and less dense water rises the world's ocean becomes density stratified because density is largely a function of temperature and salinity the movement of deep water due to density differences is called thermohaline circulation currents near the sea floors flow a slow river like masses in some places but for the most part the greatest volumes of deep water creep belong to the ocean at the ocean's floor and an almost imperceptible pace and the Coriolis effect gravity and friction shaped the direction and the volume of surface currents and the thermohaline circulations and that does wrap up ocean circulations with an overview of the lecture concepts we'll see you next time when we continue with the lectures from an introduction to oceanography from essentials of oceanography the cengage learning textbook written by Tom garrison Robert Ellis of course we're working out of the 8th edition

Channel: Dave Cocchiarella

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Keywords: Oceanography, ocean currents, El Nino, Thermohaline circulation, Dave Cocchiarella

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Length: 42min 33sec (2553 seconds)

Published: Mon Jan 01 2018